Suppose a social networking platform has the social relationship of users, stored by (user, [list of friends]) pairs as shown below:

User Friends A B C D B A C D E C A B D E

Note that the social relationship is symmetric: A is Bs friend always means B is As friend as well. Design a MapReduce program in pseudocode to find the common friends of any two users. For example, the common friends of (B, C) are {A, D, E}.